Forking

Let’s play around with fork(). This system call creates a copy of the currently running process, returning twice: once to the parent, once to the child. Here’s an example:

#include <stdio.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
    int x = 0;

    pid_t pid;

    // Non-production code: not checking for errors
    if ( (pid = fork()) == 0){
        printf("I'm in the child. x=%i", --x);
    }

    printf("Goodbye. x=%i\n", ++x);
}

After compiling and running this program, what are some possible results that get printed out (to standard output) ?

Well, here are a few examples:

I'm in the child. x=-1
Goodbye. x=0
Goodbye. x=1

or

I'm in the child. x=-1
Goodbye. x=1
Goodbye. x=0

or even

Goodbye. x=1
I'm in the child. x=-1
Goodbye. x=0

This last example is possible because the kernel launched the child process first. In fact, you cannot guarantee that they parent will execute first (at least not that I’m aware of).